A molecule of water consists of an oxygen atom
in the center, bonded to two hydrogen atoms in a
bent shape (like a banana or a boomerang).
Because of oxygen’s small size and larger number
of protons, it has a very strong ability to
attract the electrons of nearby atoms, including
the hydrogen that it is bonded to. This leads
the oxygen to be partly negatively charged and the
hydrogen partly positively charged, so the
whole molecule behaves like a tiny magnet
(polar). When water molecules are near each
other, they arrange themselves so that the oxygens
are positioned near hydrogens on other molecules
and they strongly stick together.
This intermolecular force is strong
enough that it is called “hydrogen bonding”.
No electrons are shared or exchanged, like in
covalent or ionic bonding, which makes it about
1/20th the strength of a covalent bond.
Hydrogen bonding occurs with any molecule that has
a hydrogen bonded to an electronegative atom:
nitrogen, oxygen, fluorine, chloride, bromine or
iodine. However, this force has the strongest
effects on the properties of water, with the
molecules small size and two hydrogens that can
participate in the bonding.
Because of the strong hydrogen
bonding, water has properties that are different
when compared to other liquids:
- - Water strongly interacts with other
polar molecules that have partial charges,
allowing to dissolve many different materials like
salts and many organic compounds. This also allows
it to stick to polar surfaces like glass and be
drawn up thin tubes in capillary action.
- - Water also strongly sticks to itself,
giving it high surface tension. This is why
water droplets form (rather than spreading out
thinly over a surface).
- - Water takes a lot of energy to disrupt
this hydrogen bonding, so water has a high heat
capacity and absorbs a lot of energy in order
to change temperature or evaporate (from liquid to
- - When becoming solid, water molecules
arrange themselves in a very specific lattice
that is less dense and more rigid than its liquid
form. This allows ice to float in water,
rather than sink.
These properties are essential for life on Earth,
at least as we know it. Much of the Earth is
covered in water, which regulates the surface
temperature. Since ice floats rather than sinks,
these top layers of ice shield lakes, seas and
oceans (and all the creatures living in them) from
completely freezing during cold weather. All
organisms use water to dissolve and move minerals
and nutrients around their cells and their bodies.
Water also regulates their internal temperature
too. Can you think of other ways that water is
essential for life?
Water is a weird hybrid that bonds in a fashion
that isn't entirely covalent (atoms sharing the
electrons) or ionic (the anion, in this case
oxygen, hogging all the electrons). What results
is that water is a very polar molecule. This has
the following consequences:
1. The boiling point of water is high relative
to other similar compounds (e.g. hydrogen
sulfide). This is why water is liquid at room
temperature. Based on its molecular weight, it
should be a gas.
2. The crystal structure of solid water (i.e.
ice) places the water molecules farther from
each-other than they can pack as a liquid. As a
result, liquid water is denser than frozen water.
3. Since water is polar but not ionic, it can
easily dissolve ionic substances like salts, but
can even dissolve covalently-bonded compounds like
sugars, and even then has some limited ability to
dissolve non polar substances. This makes water an
almost universal solvent.